The present invention relates generally to an explosive device useful for cutting tubing in oil and gas well drilling applications and a method for assembling the device. More particularly, the device comprises an explosive charge in a zirconia casing, an electrically activated means for detonating the explosive charge, and a housing for attaching the casing and detonation means to a drill string. The explosive charge is then lowered to a desired point in an oil or gas well and detonated by passing an electric charge down the drill string to the detonation means. The method of assembly comprises grounding the explosive charge and detonator at all times during assembly. This minimizes the possibility of detonation during assembly due to static electric charges or stray electric currents in the components of the device. Detonation of the explosive charge or detonation means during assembly could cause serious injury to a person assembling the device.
Conventional devices for cutting tubing in oil or gas wells have used either mechanical cutters or explosive charges to separate the tubing into two segments. Mechanical cutters are lowered into the well to the desired point, and generally include teeth or other cutting elements that rotate or otherwise move and cut through the tubing to separate it. Explosive-charge cutting devices, on the other hand, use a shaped explosive charge that is lowered to the desired point in the well and then detonated. The explosive charge is shaped so that it causes the tubing to separate at the desired point when it is detonated.
Conventional explosive-charge tubing cutters typically enclose the explosive charge in a casing which is attached to a drill string and includes a means for detonating the explosive charge that is activated by an electric current. The electric current is provided by an external circuit and controlled by an operator at the top of the well. The electric current is passed down the drill string by means of a cable to the tubing cutter when the explosive device is at the proper position to cut the tubing. The electric current causes the detonation means, usually a blasting cap, to detonate, which in turn causes the explosive charge to detonate. Ideally, the tubing cutter, except for the explosive charge and its casing, can then be retrieved from the well.
Many conventional explosive-charge tubing cutters use a steel or cast iron casing for the explosive charge. These metal casings have the disadvantage that when the explosive charge is detonated the casing breaks into large pieces. These pieces can then jam or plug parts of the drill string and may make retrieval of the drill string difficult by jamming between the well casing and the drill string.